It is generally known that, inter alia, particulate matter (PM), which has carbon as its main component, and ash, which is composed of uncombusted components, are present in the exhaust gas discharged from internal combustion engines and are a cause of air pollution. Due to this, the regulations on particulate matter emission levels, along with those on harmful components present in exhaust gas, e.g., hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx), are becoming more rigorous with each passing year. Art has thus been proposed for capturing and thereby removing this particulate matter from exhaust gases.
For example, a diesel particulate filter (DPF) for capturing this particulate matter has been disposed in the exhaust passage of diesel engines. In addition, gasoline engines discharge a certain amount of particulate matter with their exhaust gas, although this amount is smaller than for diesel engines, and as a consequence in some instances a gasoline particulate filter (GPF) is also installed in the exhaust passage of gasoline engines. Particulate filters having a wall-flow structure are known here; these are constructed from a large number of cells composed of a porous substrate wherein the inlets and outlets of the large number of cells are blocked in alternation. In a wall-flow particulate filter, the exhaust gas that has entered through a cell inlet passes through the porous cell walls provided as partitions and is discharged towards and at a cell outlet. While the exhaust gas is traversing the porous cell walls, the particulate matter is captured and removed at the wall surface and in the pores in the interior of the wall. Patent Literature 1 is an example of this type of prior art.